Add Checkbox ETTN receipts for Nova Poshta COD waybills

Cashier creates an ETTN receipt in Checkbox bound to the TTN with payment
control; Checkbox fiscalizes it itself when the parcel is paid for.

- cash_registers (Fernet-encrypted license key / PIN) and receipts tables
- Checkbox HTTP client + stub (ETTN does not work on test registers)
- two-phase create via ARQ job, timeout reconciliation, cron status polling
- /receipts and /cash-registers API, audit records
- dashboard: per-order and bulk create, prepayment, cancel; cash registers page

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/core/config.py

@@ -65,6 +65,14 @@ class Settings(BaseSettings):
     # --- Nova Poshta ---
     nova_poshta_api_key: str = ""
 
+    # --- Checkbox ---
+    checkbox_base_url: str = "https://api.checkbox.ua"
+    checkbox_client_name: str = "lux_fiscal"
+    checkbox_client_version: str = "0.1.0"
+    # Стаб вместо реального Checkbox: ЕТТН работает только на боевой кассе,
+    # поэтому локально весь цикл прогоняется через стаб. В проде запрещено.
+    checkbox_use_stub: bool = False
+
     @property
     def cors_origins(self) -> list[str]:
         """Список разрешённых origin'ов из строки через запятую."""